(1) Field of the Invention
The present invention relates to a voltage detecting device, particularly to a voltage detecting device for detecting a voltage across a plurality of secondary batteries connected in series, which constitute an on-vehicle high-voltage battery.
(2) Description of the Related Art
Recently, a hybrid electric-internal combustion vehicle (hereinafter, HEV), which travels by using both an engine and an electric motor, has been widely used. The HEV has two kinds of batteries consisting of a low-voltage (such as about 12V) battery for starting an engine and a high-voltage battery for driving an electric motor. The high-voltage battery provides a high voltage by connecting a plurality of secondary batteries such as nickel-hydrogen batteries or lithium batteries, each said secondary battery being an electric cell or a unit cell.
As for the high-voltage battery described above, a voltage of each secondary battery, that is, a state of charge (hereinafter, SOC) of each secondary battery varies widely (i.e. becomes uneven) while the secondary battery repeats charge and discharge. During the charge and discharge of the secondary battery, from the viewpoints of durability and safety of each secondary battery, it is necessary to inhibit the charge at a time point when the secondary battery having the highest SOC reaches a predetermined upper limit SOC and to inhibit the discharge at a time point when the secondary battery having the lowest SOC reaches a predetermined lower limit SOC. Accordingly, when the SOC of each secondary battery becomes uneven, a usable capacity of the secondary battery is substantially reduced. That is, as for the HEV, this undesirable phenomenon makes so-called an assist and regeneration insufficient, wherein the assist means a supplement of battery energy with respect to gasoline upon hill-climbing and the regeneration means regeneration of energy to the battery, thereby causing deterioration in a vehicle dynamic characteristic or fuel efficiency. Therefore, it becomes necessary to detect a voltage of each secondary battery in order to equalize the SOC of each secondary batter.
So far, as a voltage detecting device which detects a voltage of each secondary battery that is a component of the high-voltage battery described above, a device disclosed in Japanese Patent Application Laid-Open No. 2003-243044 has been proposed. In this voltage detecting device, a high-voltage battery is divided into a plurality of modules and a voltage of each secondary battery in each module is detected by a central processing unit (hereinafter, CPU) disposed per module. With such a construction, voltage values across a plurality of respective secondary batteries can be detected simultaneously and a withstanding voltage of an element that is used to detect the voltage of the secondary battery can be reduced.
In the conventional voltage detecting device described above, due to unevenness of a reference voltage used for detecting a voltage of each module and unevenness of accuracy of an A/D converter, a detection error takes place among the modules. If the detection error among the modules is large, an equalizing error takes place when the SOC of each secondary battery is equalized and therefore, each secondary battery could not be used efficiently. If high-accuracy reference voltage and high-accuracy A/D converter are used, this error can be removed. However, in such a case, it is necessary to have high-accuracy reference voltage and high-accuracy A/D converter for the respective modules, thereby causing a cost-up.